The present invention generally relates to circuits for reducing intermodulation distortions in frequency-modulated signals, and more particularly to an intermodulation distortion reducing circuit having a simple circuit construction and capable of reducing intermodulation distortion in a frequency-modulated signal.
Generally, apparatuses which record an information signal comprising a color video signal and an audio signal onto a rotary recording medium (hereinafter simply referred to as a disc) as existence or non-existence of pits, that is, as variation in convex and concave configuration, and reproduce the recorded information signal from the disc by a system using variation in the electrostatic capacitance between a reproducing element and the disc or a system using a light beam, have been reduced to practice. However, when recording the information signal onto the disc and forming the disc, it is impossible to form the pits with accurate convex and concave configuration, and the shapes of the pits are not perfect. For example, in the electrostatic capacitance type system, the recorded signal is reproduced by detecting the variation in the electrostatic capacitance between an electrode of a reproducing stylus and the disc, because the electrostatic capacitance varies according to variation in the convex and concave configuration on the surface of the disc. In such an electrostatic capacitance type system, the variation in the electrostatic capacitance is not affected by the shape of a concave region which is considerably separated from the electrode of the reproducing stylus, and the waveform of the reproduced signal is affected by the shape of a convex region which is contiguous to the electrode of the reproducing stylus. Accordingly, the signal reproduced from this type of a disc includes distortion according to the abnormal shape of the convex formed on the surface of the disc.
If the information signal is recorded onto the disc according to a constant angular velocity system, the size of the pits formed on the disc becomes smaller towards the inner periphery of the disc, because the linear speed decreases towards the inner periphery of the disc. Hence, generation of an intermodulation distortion is especially notable in the signal reproduced from the inner peripheral part of the disc.
On the other hand, the information signal recorded onto the disc is obtained in the following manner. That is, a frequency-modulated audio signal, which is obtained by frequency-modulating a carrier by an audio signal, is multiplexed with a color video signal in different bands. A carrier of a predetermined frequency is frequency-modulated by the multiplexed signal, and the information signal is recorded onto the disc in the form of this frequency-modulated multiplexed signal.
When the intermodulation distortion is introduced in the frequency-modulated multiplexed signal reproduced from the disc, an intermodulation distortion component of one of the color video signal and the frequency-modulated audio signal or intermodulation distortion components of both the color video signal and the frequency-modulated audio signal, may become mixed within the band of the other signal or mixed into their mutual bands. In this case, if such a frequency-modulated multiplexed signal is demodulated, degradation is introduced in the picture quality of the reproduced picture, the sound quality of the reproduced sound, and the like, due to beat interference introduced by the above mixture of the intermodulation distortion components.
Accordingly, it is necessary to reduce and eliminate the intermodulation distortion before the reproduced frequency-modulated multiplexed signal is demodulated. However, although the intermodulation distortion in the high frequencies can be eliminated by simply passing the reproduced frequency-modulated multiplexed signal through a lowpass filter, it is impossible to eliminate the intermodulation distortion mixed within the required side-bands. Thus, a circuit for reducing the intermodulation distortion cannot be realized by use of a lowpass filter alone.